BC mode allows certain automatic type conversions to occur, as well as automatic selection of the first member of a node-set.
BC模式允许进行某些自动类型转换和自动选择节点集的第一个成员。
2
If the BC feature is available, you can set version 1.0 to be in effect for an element with expression evaluation, eliminating worry about type mismatches.
BC mode impacts numeric comparisons in two ways: allowing selection of the first member of a set and obtaining NaN for a non-numeric value, rather than throwing a type error.